Guys' Weird New Habits: Why? Why?

First came chest man-scaping, made famous in The 40-Year-Old Virgin (above). Then earlier this year, Jay-Z and P. Diddy reportedly had "brozilians"—the male equivalent of the Brazilian wax. Women everywhere let out a big now-you-know-how-it-feels whoop of joy, followed by a resounding "Huh?!" Guys are so good at being hairy—what gives? Blame the evolution of metrosexuality and the mainstreaming of porn: That hair-free aesthetic is catching on with guys, too. "My girlfriend loves it for the same reason I like it on her: It's soft and smooth and clean," says Evan, 29, a music producer in New York City. It doesn't hurt things in bed, either. "Waxing can't turn bad sex into great sex," he says, "but the extra sensitivity you both feel can make good sex even better." It's a whole new ball game. (Sorry, we couldn't resist.)

Why Are Young Guys Undergoing Vasectomies?

Used to be that men of a certain age got snipped when they were done having kids. But now a small but increasing number of childless guys in their twenties—ones who aren't even in relationships—are choosing to have the surgery too. "Unlike the spur-of-the-moment tattoo, it's usually a decision they've thought long and hard about," says Paul Shin, M.D., a urologist in Washington, D.C., who specializes in male fertility issues. "I see guys who think the world is overpopulated and want to reduce their environmental impact on the planet."

Douglas Stein, M.D., of Tampa, Florida, who performs about 2,000 vasectomies a year and is known in the field as the "King of Vasectomies," sees another theme among his patients: "The way the world is, I just don't want to bring a kid into it." And, yes, some guys want to preserve their freedom—call it the Knocked Up ripple effect. Days after we spoke, Dr. Stein performed three vasectomies, all on men in their early twenties. One agreed to get on the phone with us—Eddie, 23, a college student in St. Petersburg who recently returned from a tour in Iraq. "I did it because older guys I work with got in a lot of trouble from having kids unplanned with their girlfriends," he says. "They had to stay in the military to support the kids, and their lives got sidetracked." At that point in the interview, he excused himself: "Sorry, I've got to go get on ice."

Jeremy Youngman, a 32-year-old artist in Downer's Grove, Illinois, who had a vasectomy at 27 because he feels there are already enough kids in the world, is so committed that not even dream girl Christina Aguilera could get him to go back. "If Christina divorced her husband," he says with feeling, "and we fell in love and got married and she asked me to reverse it, I would say, Sorry, baby.'"

Why are guys Wearing Skinny Jeans?

Blame boy rockers. This look originated with the Pete Wentz/Joel Madden crowd. "Maybe tight pants are the male Wonderbra, in the way that they show off your junk," suggests Grant, 32, a journalist in New York City who wears them. And do the ladies like the view? "Actually," he says, "I see more gay guys looking than girls!"

Why Are Guys So Amused by "Braining"?

Men love to play practical jokes on one another—and the latest prank is "braining." If you're not familiar with it, here's how it works: A guy falls asleep after drinking. His buddy—the "brainer"—takes out just his testicles, which, on their own, resemble a mini brain, and places them next to the snoozer's face. (The positioning of the big kahuna varies.) Then he points a camera downward, snaps a photo of the spectacle and posts it on his Facebook and MySpace pages until the sites delete it. "It's basically a lighter version of the stuff guys do when their friends pass out drunk, like shaving off their eyebrows or drawing on their faces with permanent marker," explains brainer Brian B. "Except it's better because you don't need any gear, other than what you've got."

Why Are Guys Still Taking Such Risks Just to Get Laid?

New York's ex-governor Eliot Spitzer is a smart man, right? Like, Jeopardy-champion smart. Yet you'd have to be one fry short of a Happy Meal to think you could possibly get away with cheating the way he did.

The risks Spitzer took look especially irrational given that deceiving a wife or girlfriend is actually harder than ever, if you think about it. "I can't imagine trying to pull off cheating now," says David, a thirtysomething who says he was unfaithful to every woman he dated in his teens and twenties, before cell phones, texting and e-mail became ubiquitous. "Technology has made it tough because you're always supposed to be online and reachable." Sites like womansavers.com, a worldwide database of allegedly unfaithful men, make fooling around even harder.

So what's going on? Paradoxically, the fact that it's easier than ever to get caught may entice some men, says WomanSavers founder Stephany Alexander. "It's part of the thrill to see if they can get away with it." In the end, though, it comes down to this one thing: "Men who cheat tend to be narcissistic," says Stephanie Buehler, Psy.D., a sex therapist in California who counsels couples. "They think that everyday rules do not apply to them, and they think that other people are not as clever as they are. So they may really believe that they're so smart and sly that their partner will never figure out that they're cheating." Never say never.

Why Are Guys Obsessed With Making Sex Tapes?

"This generation of guys is the first that grew up watching porn at home, hiding it from their parents," says Erika Lust (yes, that's what she goes by), cofounder of the feminist erotic-movie company Lust Films. "So they have a special relationship with taped sex. Some of them think something on-screen is automatically sexier than in reality."

With today's cell-phone cameras and webcams, anyone can be an amateur pornographer. Jacob, 27, a graphic designer in Boston, used his cell to tape himself and his girlfriend fooling around in a bar bathroom. "Making your own masturbation material is probably the number-one reason guys tape sex," says Jacob. "We can rely on the mental hard drive, but having video is so much better for us."

But beware the number-two reason guys tape sex: "I would estimate that if a girl gets naked for a camera, there is an 80 percent chance it will be seen by other guys, if not posted online eventually," says Matt, a 33-year-old entrepreneur in New York, who has had two male friends share their sex tapes with him. "Guys do it to show off and to prove their masculinity." If you're going to appear in a video, make sure you hang on to the footage.

Why Are Guys Saying No to Casual Sex?

We always figured we could count on three certainties in life: death, taxes and the male libido. Question is, why are men (the ones who aren't cheating) suddenly turning down flings—a trend we're hearing lots of women gripe about? In part, it's because more women, whether they want a date or booty call, are making the first move (a very good thing). Which means—fair's fair—that it's our turn to hear the occasional no.

Men are also realizing it's OK to hold off. "In the past, when a young man turned down sex, other men questioned his sexual identity," says Mary Jo Rapini, a psychotherapist in Houston. "But there isn't the social stigma that there was five years ago. Thanks to STD education, men feel more comfortable waiting."

Henry, 29, a musician in Chicago, says sometimes he's chaste because it's less messy. "Women talk a big game about casual sex, but I don't believe they're as good at it as they think they are," he contends. "They initiate and they're all playful and casual, and the next morning it's like, What are we doing today, baby?' Now I'm very careful who I say yes to."

So these days guys wear tight jeans, get bikini waxes, worry about birth control and are willing to wait for sex. Hey, maybe men aren't so different from us after all.